Auto merge of #146666 - Zalathar:rollup-m2b8low, r=Zalathar

Rollup of 14 pull requests

Successful merges:

 - rust-lang/rust#142807 (libtest: expose --fail-fast as an unstable command-line option)
 - rust-lang/rust#144871 (Stabilize `btree_entry_insert` feature)
 - rust-lang/rust#145071 (Update the minimum external LLVM to 20)
 - rust-lang/rust#145181 (remove FIXME block from `has_significant_drop`, it never encounters inference variables)
 - rust-lang/rust#145660 (initial implementation of the darwin_objc unstable feature)
 - rust-lang/rust#145838 (don't apply temporary lifetime extension rules to non-extended `super let`)
 - rust-lang/rust#146259 (Suggest removing Box::new instead of unboxing it)
 - rust-lang/rust#146410 (Iterator repeat: no infinite loop for `last` and `count`)
 - rust-lang/rust#146460 (Add tidy readme)
 - rust-lang/rust#146552 (StateTransform: Do not renumber resume local.)
 - rust-lang/rust#146564 (Remove Rvalue::Len again.)
 - rust-lang/rust#146581 (Detect attempt to use var-args in closure)
 - rust-lang/rust#146588 (tests/run-make: Update list of statically linked musl targets)
 - rust-lang/rust#146631 (cg_llvm: Replace some DIBuilder wrappers with LLVM-C API bindings (part 3))

r? `@ghost`
`@rustbot` modify labels: rollup

Author: bors

src/tests/intro.md

@@ -70,10 +70,12 @@ package tests:
 
 Tidy is a custom tool used for validating source code style and formatting
 conventions, such as rejecting long lines. There is more information in the
-[section on coding conventions](../conventions.md#formatting).
+[section on coding conventions](../conventions.md#formatting) or the [Tidy Readme].
 
 > Examples: `./x test tidy`
 
+[Tidy Readme]: https://github.com/rust-lang/rust/blob/master/src/tools/tidy/Readme.md
+
 
 ### Formatting